1. Field of the Invention
The invention relates to a frequency mixer circuit, and more particularly to a down-conversion frequency mixer circuit.
2. Description of the Related Art
FIG. 1 shows a conventional Gilbert cell mixer circuit including a balun 11, a radio-frequency (RF) voltage-to-current (V-to-I) converter 12, and a mixer 13.
The balun 11 has a local oscillator (LO) terminal to receive a single-ended LO voltage signal VLO with a frequency fLO and converts the single-ended LO voltage signal into a differential LO voltage signal VDLO.
The RF V-to-I converter 12 has an input terminal to receive a single-ended RF voltage signal VRF with a frequency fRF, and performs voltage-to-current conversion on the single-ended RF voltage signal VRF to generate a differential RF current signal IDRF. The RF V-to-I converter 12 includes inductors L1, L2, transistors M1, M2, and a capacitor C1 coupled between a drain terminal of the transistor M1 and a gate terminal of the transistor M2.
The transistor M1 has a gate terminal receiving the single-ended RF voltage signal VRF, a grounded source terminal, and a drain terminal coupled to a first terminal of the inductor L1. The transistor M2 has a grounded source terminal and a drain terminal coupled to a first terminal of the inductor L2. The differential RF current signal IDRF is generated at second terminals of the inductors L1, L2, and includes a current ID1 flowing through the inductor L1.
The mixer 13 is coupled to the balun 11 through capacitors C2, C3 for receiving the differential LO voltage signal VDLO therefrom, is coupled to the inductors L1, L2 for receiving the differential RF current signal IDRF, and has a pair of output terminals to output a differential intermediate-frequency voltage signal VDIF with a frequency fIF equal to a difference between fRF and fLO (i.e., fIF=fRF−fLO).
When the conventional Gilbert cell mixer circuit is operated at radio frequency, the current ID1 is approximate to a current ID3 flowing through the drain terminal of the transistor M3 (i.e., ID1≈ID3). The Gilbert cell mixer circuit has a conversion gain (CG) as given in the following equation:
  CG  =            VDIF      VRF        =                            ID          ⁢                                          ⁢          1                          ID          ⁢                                          ⁢          3                    ×              g                  m          ⁢                                          ⁢          1                    ×      R      ×      A      where gm1 is transconductance of the transistor M1, R is equivalent output impedance at the second terminal of the inductor L1, and A is a conversion gain of the mixer 13.
The aforementioned conventional Gilbert cell mixer circuit does not have sufficient conversion gain since the current ID1 is approximate to the current ID3. In addition, FIG. 2 shows that the conventional Gilbert cell mixer circuit has insufficient isolations among the LO terminal, the input terminal and the output terminals. In particular, since the RF V-to-I converter 12 has only one transistor stage that includes the transistors M1, M2, signals at the LO terminal, the input terminal, and the output terminals of the mixer 13 may interfere with each other, resulting in low isolations.